This is a really bad photos as the bird was too far away for our lenses and it is heavily cropped, but for ID purposes, can anyone identify it? I thought it might be a Grey-headed Gull? We saw it at Mestel Dam near Pretoriuskop early in June.

This one has me puzzled.We saw this gull at the Groot River mouth (Nature's Valley) on January 25, 2016. I noticed it in flight over the estuary, and what caught my eye was the white underwing coverts and black-edged primaries (wing-tips): this was unlike an adult (or juvenile) Kelp Gull. The one glimpse i got of the wings from above was as the gull turned to land, the black-tipped primaries appeared to be separated from the greyish part of the wing closest to the body (if this makes sense) by a triangular white patch (sort of a three-toned upper wing going grey-white-black from body to wingtip). The gull landed reasonably close to a Kelp Gull, and I then thought maybe it is a juvenile Kelp Gull, but it appeared quite noticeably smaller than the adult bird.Anyhow; long story short: this is the best photograph I could get of the bird before it took off again:

Any thoughts? Could it possibly be an immature Grey-headed Gull? The only other photographs I could find via uncle Google suggests Black-headed Gull, but this seems unlikely.